U.S. patent application number 15/327356 was filed with the patent office on 2017-06-15 for nox reduction control method for exhaust gas aftertreatment device.
The applicant listed for this patent is ISUZU MOTORS LIMITED. Invention is credited to Daiji NAGAOKA.
Application Number | 20170167343 15/327356 |
Document ID | / |
Family ID | 55078484 |
Filed Date | 2017-06-15 |
United States Patent
Application |
20170167343 |
Kind Code |
A1 |
NAGAOKA; Daiji |
June 15, 2017 |
NOx REDUCTION CONTROL METHOD FOR EXHAUST GAS AFTERTREATMENT
DEVICE
Abstract
This NOx reduction control method is for an exhaust gas
aftertreatment device having an oxidation catalyst and an LNT
catalyst which are disposed in an exhaust pipe and repeating an
adsorption or occlusion of NOx which is executed when an air-fuel
ratio is in a lean state and a reduction of NOx which is executed
when the air-fuel ratio is in a rich state, the method including
executing a post-injection or an exhaust pipe injection and causing
HC to be adsorbed in the oxidation catalyst when an exhaust gas
temperature is low, and causing the HC which is adsorbed in the
oxidation catalyst to be desorbed and reducing an adsorbed NOx in
the LNT catalyst by raising the exhaust gas temperature during the
rich state.
Inventors: |
NAGAOKA; Daiji;
(Fujisawa-shi, Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ISUZU MOTORS LIMITED |
Tokyo |
|
JP |
|
|
Family ID: |
55078484 |
Appl. No.: |
15/327356 |
Filed: |
July 13, 2015 |
PCT Filed: |
July 13, 2015 |
PCT NO: |
PCT/JP2015/070039 |
371 Date: |
January 18, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02T 10/26 20130101;
F01N 3/204 20130101; F01N 2550/03 20130101; F01N 2900/1614
20130101; F01N 3/2006 20130101; F01N 3/0814 20130101; F01N 11/002
20130101; F01N 2430/06 20130101; F01N 2560/14 20130101; F01N 3/08
20130101; F01N 2430/085 20130101; F01N 2560/026 20130101; Y02T
10/24 20130101; Y02T 10/47 20130101; F01N 3/106 20130101; Y02T
10/40 20130101; F01N 3/0842 20130101; Y02T 10/12 20130101; F01N
3/2033 20130101; F01N 3/021 20130101; F01N 2560/06 20130101; F01N
2610/03 20130101; F01N 2900/1404 20130101; F01N 3/103 20130101;
F01N 3/0871 20130101 |
International
Class: |
F01N 3/20 20060101
F01N003/20; F01N 3/021 20060101 F01N003/021; F01N 11/00 20060101
F01N011/00; F01N 3/08 20060101 F01N003/08; F01N 3/10 20060101
F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 18, 2014 |
JP |
2014-147816 |
Claims
1. An NOx reduction control method for an exhaust gas
aftertreatment device having an oxidation catalyst and an LNT
catalyst which are disposed in an exhaust pipe and repeating an
adsorption or occlusion of NOx which is executed when an air-fuel
ratio is in a lean state and a reduction of NOx which is executed
when the air-fuel ratio is in a rich state, the method comprising:
executing a post-injection or an exhaust pipe injection and causing
HC to be adsorbed in the oxidation catalyst when an exhaust gas
temperature is low; and causing the HC which is adsorbed in the
oxidation catalyst to be desorbed and reducing an adsorbed NOx in
the LNT catalyst by raising the exhaust gas temperature during the
rich state.
2. The NOx reduction control method for the exhaust gas
aftertreatment device according to claim 1, wherein the
post-injection or the exhaust pipe injection which is executed when
the exhaust gas temperature is 200.degree. C. or lower, and wherein
the HC which is adsorbed in the oxidation catalyst is caused to be
desorbed and the NOx is reduced by raising the exhaust gas
temperature either by executing the post-injection continuously
after a main injection or by energizing a glow plug, when the
exhaust gas temperature exceeds a catalyst activation temperature
and the air-fuel ratio is caused to be in the rich state based on
an NOx adsorption amount in the LNT catalyst.
3. The NOx reduction control method for the exhaust gas
aftertreatment device according to claim 2, the method further
comprising: executing a normal reduction of NOx by supplying FTC to
the LNT catalyst either by causing the post-injection to be delayed
or by executing the exhaust pipe injection, when an HC adsorption
amount in the oxidation catalyst becomes equal to or smaller than a
threshold value.
4. The NOx reduction control method for the exhaust gas
aftertreatment device according to claim 3, the method further
comprising: causing the air-fuel ratio to be in the lean state and
adsorbing or occluding NOx when an NOx reduction amount becomes
equal to or smaller than a threshold value.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas
aftertreatment device which utilizes an NOx occlusion reduction
catalyst and more particularly to an NOx reduction control method
for an exhaust gas aftertreatment device in which hydrocarbons (HC)
are adsorbed to an oxidation catalyst or the like of the exhaust
gas aftertreatment device for use for NOx reduction.
BACKGROUND ART
[0002] DOC (Diesel Oxidation Catalyst), DPF (Diesel Particulate
Filter) and NOx occlusion reduction catalyst (LNT: Lean NOx Trap or
NSR: NOx Strage Reduction) systems have already been put into
practice as exhaust gas aftertreatment devices for diesel
engines.
[0003] An NOx occlusion reduction catalyst is such that a noble
metal catalyst such as Pt, Pd or the like and an occlusion material
having an NOx occlusion function such as an alkali metal including
Na, K, Cs and the like, an alkali earth metal including Ca, Ba and
the like or a rare earth including Y, La, Ce and the like are
carried on a catalyst carrier such as an alumina (Al.sub.2O.sub.3)
and exhibits two functions to occlude NOx and to release and purify
NOx depending on the oxygen concentration in exhaust gases.
[0004] With a purification system utilizing the NOx occlusion
reduction catalyst (hereinafter, referred to as an LNT catalyst)
which reduces NOx by using the three-way catalyst function, under a
condition in which the oxygen concentration in exhaust gases is
high (a lean air-fuel ratio) as in a normal operating state, NO in
exhaust gases is oxidated to NO.sub.2 by a noble metal catalyst
such as Pt or Pd, and the occlusion material occludes the oxidated
substances as nitrates (Ba(NO.sub.3).sub.2) to purify NOx.
[0005] When the occlusion of NOx continues, however, since nitrates
are saturated to lose the occlusion function as the occlusion
material, the operating condition is changed to form a rich state
by performing an EGR (Exhaust Gas Recirculation), a post-injection
of fuel or an exhaust pipe injection of fuel under a low oxygen
concentration condition (a rich air-fuel ratio), so that fuel is
reduced on the noble metal catalyst to thereby produce CO, HC,
H.sub.2 in the exhaust gases so as to reduce released NOx for
purification.
[0006] In this way, in the purification system using the LNT
catalyst, the three-way catalyst function works in which NOx is
adsorbed or occluded when the air-fuel ratio is in a lean state
(under the high oxygen concentration condition), and the NOx
adsorbed or occluded are disorbed or released from Ce or Ba during
the rich state, so that HC, CO and NOx in the exhaust gases become
harmless by the three-way catalyst function.
PRIOR ART LITERATURE
Patent Literature
[0007] Patent Literature 1: JP-A-2009-002179
[0008] Patent Literature 2: JP-A-2001-050034
[0009] Patent Literature 3: JP-A-2008-240704
SUMMARY OF THE INVENTION
Problem that the Invention is to Solve
[0010] At this time, with regard to released NOx, in case the
amounts of HC and CO which are necessary for the required reaction
are not enough, part of NOx is not reduced, and the untreated. NOx
is released as it is.
[0011] Normally, HC are dosed to exhaust gases as a result of
performing a post-injection in the cylinders or dosing HC into the
exhaust pipe and are dissociated by temperatures or a catalyst
reaction in the DOC to be supplied to the catalyst.
[0012] However, when the temperatures of the exhaust gas and the
catalyst are low (equal to or lower than 200.degree. C.), since it
takes some time until supplied unburnt fuel is dissociated into HC,
the NOx reduction efficiency is reduced during the rich state and
NOx tends to slip easily.
[0013] When the temperature becomes high to some extent (for
example, 250.degree. C. or higher), HC are dissociated at higher
speeds so as to more easily contribute to the reduction of NOx,
whereby the NOx slip is reduced during the rich state.
[0014] The switching between the lean and rich operations is
performed as follows. An NOx adsorption amount is estimated based
on a detection value of an NOx sensor provided at the inlet and
exit of the LNT catalyst. Alternatively, an NOx adsorption amount
is estimated by obtaining an NOx amount based on a map from an NOx
concentration which is based on an operating state of the engine,
that is, the amount of injection of fuel and an exhaust gas flow
rate, and integrating the obtained NOx amount according to the
operating state. Then, when the NOx adsorption amount exceeds a set
value, the operating state is switched from the lean operating
state to the rich operating state, and when the NOx reduction
amount becomes equal to or smaller than a threshold value, the
operating state is switched from the rich operating state to the
lean operating state.
[0015] However, as described above, when the temperature of the
exhaust gas is equal to or lower than 200.degree. C. which is the
catalyst activation temperature, there is caused a problem that
even when the operating state is switched from the lean operating
state to the rich operating state, the NOx reduction cannot be
effected sufficiently.
[0016] Accordingly, an object of the present invention is to solve
the problem described above and to provide an NOx reduction control
method for an exhaust gas aftertreatment device which can execute
an NOx reduction without any problem even when the temperature of
the exhaust gas is low.
Means for Solving the Problem
[0017] With a view to achieving the object, according to the
present invention, there is provided an NOx reduction control
method for an exhaust gas aftertreatment device having an oxidation
catalyst and an LNT catalyst which are disposed in an exhaust pipe
and repeating an adsorption or occlusion of NOx which is executed
when an air-fuel ratio is in a lean state and a reduction of NOx
which is executed when the air-fuel ratio is in a rich state, the
method including executing a post-injection or an exhaust pipe
injection and causing HC to be adsorbed in the oxidation catalyst
when an exhaust gas temperature is low and causing the HC which is
adsorbed in the oxidation catalyst to be desorbed and reducing an
adsorbed NOx in the LNT catalyst by raising the exhaust gas
temperature during the rich state.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is a schematic diagram showing a device for executing
an NOx reduction control method for an exhaust gas aftertreatment
device of the present invention.
[0019] FIG. 2 is a flowchart showing the NOx reduction control
method for an exhaust gas aftertreatment device of the present
invention.
MODE FOR CARRYING OUT THE INVENTION
[0020] Hereinafter, a preferred embodiment of the present invention
be described in detail based on the accompanying drawings.
[0021] FIG. 1 shows an exhaust gas aftertreatment device 10 which
utilizes an LNT catalyst.
[0022] A turbocharger 11 and an EGR pipe 12 are connected to an
intake and exhaust systems of an engine E, whereby air which is
taken in from an air cleaner 13 is compressed by a compressor 14 of
the turbocharger 11 and is then sent under pressure to an intake
passageway 15 so as to be supplied into the engine E from an intake
manifold 16 of the engine E. An intake valve 17 which controls the
amount of air supplied to the engine E is provided along the intake
passageway 15.
[0023] Exhaust gas discharged from the engine E is discharged from
an exhaust manifold 18 to a turbine 19 of the turbocharger 11 to
drive the turbine 19 and is then discharged into an exhaust pipe
20.
[0024] The EGR pipe 12 is connected to the intake manifold 16 and
the exhaust manifold 18, and both an EGR cooler 21 for cooling
exhaust gases which flows from the exhaust manifold 18 to the
intake manifold 16 and an EGR valve 22 for controlling an EGR
amount are connected to the EGR pipe 12.
[0025] In the exhaust gas aftertreatment device 10, an exhaust pipe
injector 23 is provided downstream of the turbine 19 on the exhaust
pipe 20, and a DOC (Diesel Oxidation Catalyst) 25, an LNT catalyst
26 and a DFP 27 are canned sequentially in a canning container 24
which is formed downstream of the exhaust pipe injector 23 on the
exhaust pipe 20.
[0026] A pre-DOC exhaust gas temperature sensor 28 is provided
upstream of the DOC 25, and a post-DOC exhaust gas temperature
sensor 29 and an NOx sensor 30 are provided on an entrance side and
an exit side of the LNT catalyst 26, respectively.
[0027] An overall operation of the engine E is controlled by an ECU
32. The ECU 32 includes an NOx adsorption amount estimating means
33, an adsorption amount estimating means 34 and an NOx reduction
amount estimating means 35.
[0028] The ECU 32 executes a lean cycle in which the LNT catalyst
26 is caused to occlude NOx with an air-fuel ratio staying in a
lean state and a rich cycle in which, when an NOx occlusion rate is
reduced, NOx are reduced for purification with the air-fuel ratio
staying in a rich state by executing a post-injection in cylinders
or injecting fuel HC in a pulsating fashion by using the exhaust
pipe injector 23 shown.
[0029] In switching between the lean cycle and the rich cycle, the
NOx adsorption amount estimating means 33 estimates an amount of
NOx adsorbed by the LNT catalyst 26 during a lean combustion, and
when the NOx adsorption amount reaches a set value, the lean
combustion is switched to the rich combustion.
[0030] The NOx adsorption amount estimating means 33 obtains an
amount of NOx which is released based on a map from an NOx
concentration which is based on the operating state of the engine
and an exhaust gas flow rate, and integrates the NOx amount so
obtained to estimate an NOx adsorption amount at the LNT catalyst
26 or calculates an NOx adsorption amount based on a detection
value of the NOx sensor 30.
[0031] The ECU 32 controls the combustion of the engine based on
the lean cycle when the NOx adsorption amount is smaller than the
set value and controls the combustion of the engine based on the
rich cycle when the NOx adsorption amount is equal to or greater
than the set value.
[0032] In the present invention, in the lean cycle, the ECU 32
executes the post-injection or activates the exhaust pipe injector
23 to inject fuel when an exhaust gas temperature detected by the
pre-DOC exhaust gas temperature sensor 28 is lower than a catalyst
activation temperature (approximately 200.degree. C.).
[0033] By doing so, unburnt fuel is adsorbed by the DOC 25, during
which the unburnt fuel is so adsorbed while being dissociated to HC
in the DOC 25. This HC adsorption amount is calculated by the HC
adsorption amount estimating means 34 based on a post-injection
amount or an amount of fuel injected by the exhaust pipe injector
23.
[0034] In the rich cycle, the NOx reduction amount estimating means
35 calculates an NOx reduction amount when NOx are reduced by HC,
and when the NOx reduction amount so calculated becomes equal to or
smaller than a threshold value, the rich cycle is switched to the
lean cycle.
[0035] In the normal rich cycle, when the exhaust gas temperature
is low, since it takes some time until being dissociated into HC,
the NOx reduction efficiency at the catalyst is reduced, and the
slip of NOx tends to takes place easily.
[0036] In the present invention, when the exhaust temperature is
low, fuel (HC) is supplied into the exhaust pipe 20 by means of a
post-injection or an exhaust pipe injection to be adsorbed by the
DOC 25. The HC which are adsorbed in advance by the DOC 25 are
desorbed from the DOC 25 when the exhaust gas temperature exceeds
200.degree. C. to be used easily for NOx reduction, and the
reduction of NOx progresses even at low exhaust gas temperatures
during the rich state, whereby the NOx slip can be reduced.
[0037] In this way, the reduction of NOx when the exhaust gas
temperature is low during the rich state can be executed by making
use of the HC which are adsorbed in advance by the catalyst, and
therefore, the engine is allowed to devotedly increase the exhaust
gas temperature and produce a rich air-fuel ratio, whereby an
improvement in reduction efficiency by increasing the temperature
can be expected.
[0038] The DOC 25 and the LNT catalyst 26 act to adsorb NOx and HC
when the exhaust gas temperature is so low as to be equal to or
lower than 200.degree. C. Accordingly, in a lean combustion, HC are
supplied into exhaust gases when the exhaust gas temperature is
low, and the HC so supplied are caused to be adsorbed mainly by the
DOC 25, and part of the which is not adsorbed by the DOC 25 is
caused to be adsorbed by the LNT catalyst 26.
[0039] In case the amount of adsorption of HC is too great, when
the exhaust gas temperature is increased, there may be a case where
the exhaust gas temperature is increased extraordinarily.
Therefore, an HC adsorption amount is estimated by the HC
adsorption amount estimating means 34, and a threshold value is
provided for the ETC adsorption amount, Then, when the TIC
adsorption amount reaches the threshold value, the post-injection
or the exhaust pipe injection is stopped to prevent an excessive
adsorption.
[0040] A rich reduction is executed when the exhaust gas
temperature exceeds the activation temperatures (for example,
200.degree. C.) of the DOC 25 and the LNT catalyst 26. The rich
reduction to be executed here is intended to increase the exhaust
gas temperature by burning fuel supplied into the cylinders by
means of a post-injection and to make the air-fuel ratio of exhaust
gas rich. Namely, since the conventional supply of HC into the
exhaust pipe is not intended, as to an injection timing, the
post-injection is moved close to a main injection (for example,
within 45.degree. BTDC). A main injection amount and a
post-injection amount are calibrated so as not to produce a
sensation of physical disorder so that torque produced during the
rich state matches torque produced when combustion is performed
normally. As this occurs, a glow plug may be energized to assist
the combustion.
[0041] When the exhaust gas whose temperature is increased reaches
surfaces of the catalysts, the movement of molecules is activated,
the HC adsorbed by the DOC 25 and the NOx adsorbed by the LNT
catalyst 26 are desorbed therefrom, and the NOx are reduced under
the rich air-fuel ratio.
[0042] When the rich combustion is performed several times, the
temperatures of the catalysts are increased to a high temperature
(250.degree. C. or higher), and all the HC adsorbed by the DOC 25
are desorbed to be used for NOx reduction. Thus, when the amount of
desorption of HC is reduced to a low level based on the HC
adsorption amount estimated by the HC adsorption amount estimating
means 34, the rich combustion method is changed, and the
post-injection is delayed (for example, the crank angle is
150.degree. BTDC) so as to supply FTC from the engine to the
catalyst. Since the catalyst is heated to the high temperature, the
dissociation of HC is promoted, whereby an NOx reduction can be
executed even by HC supplied from the engine during the rich
state.
[0043] Next, the NOx reduction control method described heretofore
will be explained based on a flowchart shown in FIG. 2.
[0044] When the control is started in step S10, in step S11, when
the pre-DOC exhaust gas temperature T1>the catalyst activation
temperature (approximately of 200.degree. C.), HC are supplied into
exhaust gases by means of a post-injection or dosing HC into the
exhaust pipe.
[0045] Next, in step S12, it is determined whether or not the
pre-DOC exhaust gas temperature T1>the catalyst activation
temperature (approximately of 200.degree. C.). If it is determined
that the pre-DOC exhaust gas temperature T1 is lower than the
catalyst activation temperature (the condition is not met; NO), the
control process is returned to step S11, where HC is continued to
be supplied into exhaust gases while integrating the HC so
supplied, and an amount of HC adsorbed by the catalyst is
estimated. In these steps S11, S12, if the HC adsorption amount
reaches the threshold value (the set value), the supply of HC is
stopped.
[0046] If it is determined in step S12 that the pre-DOC exhaust gas
temperature T1 exceeds the catalyst activation temperature
(approximately of 200.degree. C.) (the condition is met; YES), the
control process proceeds to step S13, where a rich reduction is
executed if a command to execute such a rich reduction is given
based on the NOx adsorption amount. In the execution of the rich
reduction, as to the timing of injection of fuel, since fuel is
injected not to supply HC into the exhaust pipe but to raise the
exhaust gas temperature, the post-injection is executed at a timing
which is close to the timing of a main injection (for example, the
crank angle is not greater than 45.degree.), whereby fuel is
injected almost as in an after-injection. In this case, the exhaust
gas temperature may be raised by assisting the combustion by
energizing the glow plug.
[0047] Next, it is determined in step S14 whether or not a post-DOC
exhaust gas temperature T2>a fuel dissociation temperature
(250.degree. C. or higher), and if the post-DOC exhaust gas
temperature T2 does not exceed the fuel dissociation temperature or
the HC adsorption amount does not exceed the threshold value (the
condition is not met), the control process is returned to step S13,
where the post-injection is caused to continue to raise the exhaust
gas temperature whereby NOx are reduced by using the adsorbed HC in
step S13. On the other hand, if it is determined in step S14 that
the post-DOC exhaust gas temperature T2 exceeds the fuel
dissociation temperature and that the HC adsorption amount is equal
to smaller than the threshold value (the condition is met), in step
S15, the rich combustion method is changed, so that the
post-injection is delayed (for example, the crank angle is
150.degree. BTDC) to realize the normal post-injection so as to
supply HC from the engine to the catalyst to thereby execute a rich
reduction.
[0048] Next, it is determined in step S16 whether or not the NOx
reduction amount becomes equal to or smaller than the threshold
value, and if it is determined that the NOx reduction amount does
not become equal to or smaller than the threshold value (the
condition is not met), the control process is returned to step S15,
where the rich reduction is caused to continue, whereas if it is
determined that the NOx reduction amount becomes equal to or
smaller than the threshold value (the condition is met), the
control process ends in step S17 to return to the initial step.
[0049] In this way, according to the present invention, when the
exhaust gas temperature is low, the post-injection or the exhaust
pipe injection is executed so that the DOC 25 adsorbs unburnt fuel,
whereby the adsorbed unburnt fuel is dissociated into in the
catalyst even when the ambient temperature is low. Then, in
executing the rich reduction, the exhaust gas temperature is raised
to be 200.degree. C. or higher, whereby the adsorbed HC are
desorbed so that NOx are reduced by the desorbed HC, thereby making
it possible to prevent the slip of NOx in the rich reduction when
the exhaust gas temperature is low.
* * * * *